The main goal of the Core is to generate near-saturation sequence-defined transposon mutant libraries of Yersinia pestis and Burkholderia pseudomallei. The libraries should include mutants with insertions in nearly all genes nonessential for growth on laboratory medium. Mutants from the high coverage libraries (&5 unique insertions/gene) produced initially will be used to create smaller libraries providing relatively complete, redundant (two insertions/gene) coverage of the nonessential genome. The transposons used for mutagenesis will generate lacZ gene fusions, and the libraries will thus serve as sources of both insertion mutations and reporter fusions. The Core will also work to develop methods to transfer transposon mutations from a previously completed Francisella novicida library to Francisella holarctica FSC200 to facilitate genetic studies of a highly virulent Francisella strain.